A battery exhibits decreased capacity and performance as its usage is increased. Thus, it is very important to design a battery such that its long term characteristics are maintained though it is used for a long time. The long term characteristics may be a charging capacity, a discharging capacity, a thickness and an open circuit voltage of a battery, as examples.
The long term characteristics of a battery are measured based on a cycle charging/discharging process. In the cycle charging/discharging process, a certain number of batteries are sampled from a produced battery lot, and then the sampled batteries are successively charged/discharged. If long term characteristics of the sampled batteries satisfy certain criteria at predetermined long term cycles, the corresponding battery lot is considered as being successful. For example, the corresponding battery lot is considered as being successful when a discharging capacity at 3V is 75% or above of an initial capacity at 300 cycle (300 time charging/discharging).
However, a long time is consumed for estimating long term characteristics of a battery. For example, a charging/discharging process of 300 cycles takes a long time of about 3 months. Thus, in order to measure long term characteristics of a battery based on the cycle charging/discharging process, shipment of batteries is delayed during the time for the charging/discharging process, thereby increasing burdens in stockpile.
Thus, in a conventional case, if one lot of batteries is produced, a certain number of batteries are sampled and then the batteries are shipped instantly, and then long term characteristics of the sampled batteries are evaluated to take suitable measures afterward, which is called ‘post-shipment evaluation’. If any problem is discovered as a result of evaluation of long term characteristics after batteries are shipped, the battery lot corresponding to the sampled batteries is determined as having bad long term characteristics. After that, the shipped batteries are called back, and a countermeasure for removing a factor of such inferiority of long term characteristics is studied and then taken to a battery production process. However, such a post-shipment evaluation method shows the following problems.
First, in case any specific battery lot is determined as being bad, an economic cost (e.g., a distribution cost) is consumed in retrieving batteries of the corresponding lot.
Second, in case a battery having bad long term characteristics is sold to an end user, it is substantially impossible to take a measure to the battery, for example to retrieve the battery.
Third, in case it is determined that a defect exists in a production process as a result of analysis of inferior long characteristics, long term characteristics of all batteries produced by the same production process become in question, so loss of a manufacturer is increased as much.
Fourth, if a battery with bad long term characteristics is sold and used in an electronic product, a feeling of satisfaction for the battery is deteriorated, so reliability of a battery manufacturer and a seller is also deteriorated.
Thus, there is an urgent need to a scheme capable of reliably estimating long term characteristics of batteries in the related art before the batteries are shipped.